Jeff Flinn wrote:
It appears to me that the shared_ptr<T>::px is not even structurally required, but is an optimization trading space to avoid an extra pointer dereference to the sp_counted_base_impl<P,D>::P member. Or have I missed something with my limited exposure?
Consider this example: struct A { int a; }; struct B: public A { int b; }; struct C: public A { int c; }; struct D: public B, public C {}; shared_ptr<D> p1( new D ); shared_ptr<B> p2( p1 ); shared_ptr<C> p3( p1 ); shared_ptr<A> p4( p2 ); shared_ptr<A> p5( p3 ); Now p4 and p5 are both of type shared_ptr<A>, both point to an object with a dynamic type D, but p4.get() != p5.get(). The only difference is in px. (This is an answer to the general shared_ptr question. It's reasonable to disallow serialization in this particular scenario.)